In movie projectors and in tape recorders, the function to be performed by the driving elements such as motors is generally controlled by an operational mode switch made up of either a selector switch with several positions for the various functions, or of a set of push buttons. Such operational mode switches must be operated manually, requiring a certain effort. This effort can be exercised by the operator without any difficulty, but it poses problems in the case of devices to be equipped for remote control, inasmuch as the remote control signals which in most cases arrive as impulses, must be transformed with the aid of an electro-mechanical transducer. Traditionally, solenoids have been used to that end; however, they not only render the device more costly, but also require much space and weight, due to their usually very considerable number.
In addition, a device of the type mentioned at the outset has become known, in which the motor of the device itself was used as an electro-mechanical transducer capable of engaging or disengaging, as desired, the control equipment that had been designed as a program control system. Again, a solenoid was required to reverse the coupling.